1. Field of the Invention
The present invention relates to methods of and apparatus for non-invasively monitoring heart motion and is useful for the non-invasive monitoring of cardiac functions, in particular, but not exclusively, of human hearts.
2. Description of the Related Art
In the past, methods for the non-invasive monitoring of cardiac function have included:                Mechanical methods, for example, pulse recording of the jugular carotid artery or apex cardiography.        Electrical techniques, for example, electrocardiograms (ECGs).        Imaging techniques, including echocardiology, radiography and magnetic resonance imaging (MRI).        
However, mechanical methods are inaccurate because of physical differences between subjects. For example, the intensity of heart sounds cannot be accurately measured because of the fat thickness layer differences.
Some non-invasive mechanical methods do not couple properly to the external movement generated by the heart and are therefore of little use.
The electrical techniques cannot correlate to the force of cardiac contraction and are therefore of little use, and imaging techniques are also subject to this problem. For example, an echocardiogram determines a ratio known as the “ejection fraction”, which is a measure of cardiac performance which may or may not be related to the force of the heart's contraction. In a normally functioning heart, this relationship may hold true, but this finding is unreliable because the head pressure of the cardiovascular system is unknown.
None of the above-mentioned prior methods or techniques can accurately measure the isovolumic phase of the heart cycle, which is the most important parameter to measure in identifying coronary artery disease.
In U.S. Pat. No. 5,865,759, issued Feb. 2, 1999 to the present inventor, the disclosure of which is incorporated herein by reference, there is disclosed an apparatus and method to assess cardiac function in human being which employ a sensing mechanism positioned on the thyroid cartilage in the neck against the trachea for sensing a response of the thyroid cartilage to heart function.
While this prior patent disclosed a restraining system to hold the sensing mechanism in position, it was found that the apparatus is extremely sensitive to gravity because the force resulting from the weight of the sensing mechanism and a sensor restraining system varied in dependence on the vertical and horizontal position of the subject under test.
Consequently, a large cardiac force would decouple the sensing mechanism, so that a subsequent low magnitude force would be recorded poorly or not at all. These weak forces were so poorly recorded that very large electronic amplification was used, resulting in a poor signal-to-noise ratio and the recording of mostly noise. The poor coupling resulted in false data, which showed a poor correlation between the isovolumic contraction phase and the ejection phase of the heart cycle in nominal hearts, as shown by a clinical study. Another result was that the diastolic part of the cycle could not be recorded. This is a very important phase in which the passive inflow into the ventricles occurs and data relating to this phase could indicate the elasticity of the ventricular muscle. Furthermore, this prior apparatus was difficult to operate because positioning the sensor on the thyroid cartilage was difficult as elastic members forming parts of the restraining system had to be in tension balance to prevent the sensing mechanism from being moved to one side or another of the thyroid cartilage, causing erroneous data. Also, with this prior apparatus, mechanical interference caused by the sensor restraining system and an accelerometer forming part of the sensing mechanism contacted clothing, pillows, beards, fatty neck tissue, and in the case of a short neck contacted the chest, resulting in huge errors. Because no coupling apparatus was provided in this prior system, the addition of more sensors was not possible nor could reliable data be obtained. Also, this prior system employed wire connections extending directly to a recorder, resulting in stiffness and inertial effects due to interference of the wiring with the motion of the sensor.